1. Field of the Invention
The present invention relates to image correction method and apparatus for performing a shading correction of image signals picked up by an optical detector array in a scanning unit for scanning and reading image on an original.
2. Description of the Prior Art
An apparatus for reading images using an one-dimensional optical detector array is disclosed in a second embodiment in Japanese Patent Application 59-192663 proposed by the same applicant as that of the subject application.
In this apparatus, two dimensional images are read by scanning the original line by line through scanning means including lens and an optical detector array in the vertical direction to the surface of the original, with one line on the original image focusing on the detector array through a lens system respectively and with the image thus focused being converted into successive electrical signals. The image signals thus obtained are shading-corrected after converting them into digital signals.
The shading correction of the object is carried out in order to correct dispersions of gain and offset in the direction of the scanning, which are caused by the dispersions of sensitivity of the optical detector array, dark current, fluctuations in illumination of a light source and to standardize the dispersions to values corresponding to reflection index of picture elements. More specifically, a black correction signal is subtracted from an image signal and the resulting signal is corrected by dividing it by a white correction signal.
The black and white correction signals are representative of gain and offset components of a read-out system for picture elements in each scanning direction and they are stored in two different line memories of a shading correction circuit. These signals are produced from the reference signals obtainable from the reading of black and white reference marks (plates) on the original by the detector array prior to the detection of the image on the original and they are stored as already described above. Consequently, any fluctuation in the gain and offset with time elapsed can be removed by reading the reference marks just before the reading-out of the original image.
However, when any error occurs in a correction signal, a streak-like noise parallel to the vertical scanning line is added to the image signals, which will result in the degradation in the quality of image. For this reason, unless the correction signal is sufficiently lower than the image signal, a detrimental effect will come out due to the correction. On the other hand, there is a tendency that noise sometimes enters into a signal read from the reference marks due to contamination of the reference marks as well as noise in an analog system.
In order to reduce this kind of noise in the correction signal, the correction is carried out by reading the reference marks several times and then by averaging the reference signals detected plural times for each picture element. This averaging function is carried out by line memories and adders. Namely, all of the contents of the picture elements in the line memories are first cleared and then each of the picture elements of the reference signals read from the reference marks is added to the content of each of the picture elements in the line memories and it is written into the same picture elements in the line memories.
Since the total of the picture elements of the reference marks thus added plural times is stored into the line memories, it is then divided by the number of read times, thus obtaining an averaged signal. In this case, taking the number of read times as the power of 2 permits the division to be carried out only by a bit shift.
As one problem of the shading correction circuit, such as the one described above according to the prior art, the operating speed of the line memories when reading the reference signals can be enumerated. When the image signal is corrected, only access to addresses of the line memories as well as reading data is performed during one cycle of the image signal. However, when the reading of the reference signals is performed, various operations, such as access to a memory address of the line memories, the reading of data, addition and writing of the data have to be carried out during one cycle of the image signal, a high speed function of the line memories is required.
On the other hand, the number of picture elements in one line is normally a order of 1,000 to 10,000 picture elements, so that high speed memories having a large capacity respectively are required. It may be possible to realize this kind of the line memories using the nowday techniques. However, the use of the high speed line memories having a large memory capacity will result in the increase in consumption power, the increase in manufacturing costs and degradation in reliability. In addition, the circuit efficiency is not good as it operates at high speed, only when reading the reference signals.